1. Field of the Invention
The present invention relates to fiber optic networks and multi-channel communication systems.
2. Related Art
Modern communication systems increasingly rely upon fiber optic networks to carry increasing amounts of data between sites. The use of multiple optical carriers, also called channels, over the same optical fiber increases capacity. Wavelength division multiplexing (WDM) allows multiple channels to be carried on a fiber in different carrier wavelengths. Attenuation and dispersion in an optical fiber limit the distance an optical signal can travel without amplification and/or dispersion compensation. Accordingly, optical line amplifiers are provided along a fiber link to amplify an optical signal. Dispersion compensation is used to ameliorate dispersion limitations.
FIG. 1 shows an example of a fiber optic link 100 for carrying wavelength division multiplexed optical signals between site A and site B. Site A includes wavelength division multiplexers 102, 104 optically coupled to an optical fiber 110. Site B includes wavelength division multiplex units 122, 124 optically coupled to optical fiber 110. Optical fiber 110 includes two optical line amplifiers 105, 115 coupled in series to support bi-directional optical communication over the optical fiber 110.
In multi-channel communication systems, the term “operating window” refers to a band of channels supported by an optical communication network. A channel plan defines specific wavelengths assigned to each channel within an operating window. In a long-distance fiber optic network, the operating window is dependent upon the characteristics and performance of fiber amplifiers provided along a high-speed optical link. For example, one operating window for an erbium-doped fiber amplifier covers a range of wavelengths between approximately 1530 to 1561 nanometers (nm). Wavelength division multiplexed optical signals within this erbium band are amplified by a single erbium-doped fiber amplifier.
In practice, spacing must be provided between channels within an erbium band to maintain signal separation and quality. According to one International Telecommunication Union (ITU) standard, a 100 Gigahertz (GHz) spacing is provided between channels. This 100 GHz spacing translates to a wavelength range of approximately 0.8 nm, meaning only 36 or 37 WDM channels fit within an erbium fiber band. However, if each optical carrier is modulated at high data bit rates, such as 10 Giga-bits/second (Gb/s), a 200 GHz spacing is required between channels to avoid crosstalk. As a result, only sixteen channels with 200 GHz spacing can be used effectively in an operating window within an erbium band of approximately 1530 to 1561 nm.
Proposed WDM channel plans consist of 2, 4, 8 and 16 wavelength channel plans within the erbium band. For maximum capacity, it is desirable to use a 16 wavelength channel plan. Using a 16-channel wavelength division multiplexing plan requires that an erbium-doped fiber amplifier amplify across 16 channels. Problems are encountered with respect to amplification, equalization, nonlinear interference, receiver selectivity, and transmitter stability when a crowded 16-channel plan is squeezed through a single erbium-doped fiber amplifier.
In general, erbium-doped fiber amplifier gain is not even across the erbium band. A peak exists at or near 1532 nm. In other words, erbium-doped fiber amplifier gain across an operating window is not equal for all WDM channels. Power equalization circuits can be provided to equalize gain for each WDM channel. See, the patent application by X. Mao, “Multiple Wavelength Bidirectional Lightwave Amplifier,” filed Dec. 29, 1995, application Ser. No. 08/581,746 (now pending). Power equalization circuits, however, can be complex and expensive.
Amplifying multiple channels in a single erbium-doped fiber amplifier further reduces the relative output signal power for each channel. For example, an erbium-doped fiber amplifier has a maximum pump power limited by present semiconductor technology to approximately 60-130 milliwatts. The conversion efficiency between pump power and actual signal output power within an erbium band is approximately 20%. Thus, a 100 milliwatt pump power results in a total amplifier output power of approximately 20 milliwatts. If the erbium-doped fiber amplifier must support 16 channels, output signal power is distributed across each channel and reduced to about 1.3 milliwatts per channel. A low power pump source cannot even be used.
Further, conventional multi-channel WDM systems cannot be efficiently scaled to accommodate greater numbers of channels as traffic demand increases. High port count fine WDM transmitters and receivers must be installed at the outset. Incurring such an expense can be undesirable especially if current traffic demands only require a few channels. For example, installing a 16 port fine WDM transmitter and receiver to support 16 channels in an erbium band is expensive and difficult to design. Heretofore, a 16×16 port fine WDM transmitter/receiver would be required to support conventional 16 channel plans even if all 16 channels are not used initially.
Dispersion further limits the ability of a single optical fiber to support an operating window. Three types of single-mode optical fiber are commonly used in optical fiber networks: non-dispersion-shifted (NDS) fiber, zero-dispersion-shifted fiber, and low slope dispersion-shifted fiber. A non-dispersion-shifted (NDS) fiber, such as a fiber complying with the ITU G.652 standard, is only non-dispersive at the zero point which lies outside the erbium band. Zero-dispersion shifted fiber, such as a fiber complying with the ITU G.653 standard, can shift the zero-point to within erbium band but cannot remove dispersion across 16 channels in an erbium band. When 16 wavelengths are carried over a zero-dispersion shifted fiber, non-linear effects such as four-wave mixing, result in mixing products and sidebands which diminish signal quality. Thus, to get a capacity increase over a G.653 zero-dispersion shifted fiber, conventional WDM communication requires careful wavelength selection and isolation between channel directions. This limits the ability of a zero-dispersion shifted fiber to effectively support bi-directional communication over 16 WDM channels in an erbium band. Low slope dispersion shifted fiber, such as the SMF-LS fiber announced by Corning, is purported to support an erbium bandwidth with less dispersion. Low slope dispersion shifted fibers are relatively new overall in long-distance links.
A dispersion compensation module (DCM) is often provided at an optical amplifier to manage dispersion. For example, a segment of single-mode G.652 fiber has a huge dispersion on the order of 17 picoseconds/nm-kilometer at 1550 nm. Given such dispersion, transmitters can only send optical signals for up to 60 kilometers (km.) in a G.652 fiber before the dispersion prevents the optical signals from being discriminated. Dispersion problems are exacerbated when an operating window has multiple channels as in a 16-channel erbium band operating window. A dispersion compensating device compensates for absolute or bulk dispersion (magnitude). Conventional dispersion management at a single fiber amplifier, however, does not effectively control the slope or variation of dispersion across an operating window.
What is needed is an apparatus and method for optically amplifying multiple channels across an operating window that provides a relatively flat optical gain curve. Dispersion needs to be managed across an operating window, especially for a multiple channel erbium band carrying high-speed WDM optical signals. Cheaper optical fiber types, including fibers on installed networks, need to be accommodated for a multiple channel erbium band carrying high-speed WDM optical signals. The ability to use low power pump sources for a multiple channel erbium band carrying high-speed WDM optical signals is also desirable. The ability to efficiently scale a fiber network in a modular fashion to increase capacity and accommodate increasing numbers of channels for a multiple channel erbium band carrying high-speed WDM optical signals is needed.